The invention relates to a system for processing a signal being emitted from a target signal source into a noisy environment, wherein said target signal source is located in a target signal source direction xcfx86s with regard to the position of a transducer array, the system comprising: the transducer array having M transducers for receiving said signal being mixed with noise, each of the transducers generating a corresponding transducer output signal, respectively and a beamformer for receiving and filtering said M transducer output signals in order to generate at least one output signal yxe2x80x2i(n) i=1 . . . N, said beamformer comprising filter coefficients defining a predetermined filtering characteristic, e.g. a desired look direction.
The invention further relates to a method for processing a signal being emitted from a target signal source into a noisy environment, carried out by said system.
Beamforming systems are typically used to improve the quality of a received signal by processing the signals received by an array of multiple transducers. Transducer array signal processing can be utilized to enhance the performance of the receiving system capturing the desired signal that has been emitted into a noisy environment. Beamforming methods enable steering of the maximum sensitivity (look direction) of the transducer array towards any desired signal source by changing the beamforming filter coefficients. Typical applications can be found in radio communications, radar signal processing, underwater acoustics, and speech acquisition for teleconferencing and hands-free systems.
It is known in the art that according to principle of reciprocity the signal sources and receivers are interchangeable and by changing the direction of the signal flow and replacing the receiving transducers with transmitting transducers a corresponding signal can be emitted from the transducer array to direction xcfx86s.
Different beamforming methods have been studied widely in the literature. One of the most frequently studied methods is adaptive beamforming in which the filter coefficients are adjusted according to the received signal characteristics. A well known adaptive beamforming method has been presented by Frost (Frost O. L., xe2x80x9cAn algorithm for linearly constrained adaptive array processingxe2x80x9d, Proc. IEEE, Vol. 60, No. 8, pp. 926-935, August 1972). Frost""s method has been further developed by Griffiths and Jim (Griffiths L. J., Jim C. W., xe2x80x9cAn alternative approach to linearly constrained adaptive beamformingxe2x80x9d, IEEE Trans. Antennas Propag., Vol. AP-30, No. 1, pp. 27-34, January 1982). The fundamental problem of these adaptive beamformers is that the adaptive filters are designed to cancel the noise term in the beamformer output signal but, in practice, the noise estimate also contains a component that correlates with the desired signal. Therefore, the adaptive filters do not only attenuate the noise but they also cause unpredictable distortion of the desired signal. Said correlation is typically caused by multi-path propagation, misaligned look direction, or improper modeling and variations in the propagation medium.
In order to alleviate the desired signal distortion it is more favorable to design a fixed beamforming filter that is optimized for a given application.
The advantage of a fixed beamforming implementation is that the fixed filter coefficients can be optimized based on a priori knowledge of the source and the medium as well as the desired performance criteria so that the filtering performance is deterministic.
Such a design can be formulated as a spatio-temporal filter design problem where the array geometry (transducer positions) and the temporal sampling interval define a spatio-temporal sampling grid. It is known in the art that both the spatio-temporal sampling grid and the corresponding beamformer filter coefficients can be optimized without suffering from the distortion of the desired signal (Kajala M., Hxc3xa4mxc3xa4lxc3xa4inen M., xe2x80x9cBroadband beamforming optimization for speech enhancement in noisy environmentsxe2x80x9d, IEEE Workshop on Applications of Signal Processing to Audio and Acoustics, October 1999). Such a system and method will now be described in more detail by referring to FIGS. 10 to 12.
The system according to FIG. 10 comprises a transducer array 10, which comprises several transducers 10-j, j=1 . . . M. The transducers receive a signal emitted from a signal source 20 and being mixed or superimposed with noise. Transducers can be e.g. microphones to receive and transform pressure signals into electrical signals.
Said signal source is located in a target signal source direction xcfx86s with regard to the position of the transducer array 10. As illustrated in FIG. 11 said target signal source direction xcfx86s generally represents the 3-dimensional position of the target signal source in space, e.g. in Cartesian coordinates xcfx86s=(Xs, Ys, Zs) or in spherical polar coordinates xcfx86s=(rs, xcfx86s, xcex8s), relative to the transducer array 10 which is in FIG. 11 assumed to be located in the center of the coordinate system. If signals come from the far field, they can be modeled as plane waves. In that case, the distance rs becomes redundant and the signal source direction xcfx86s reduces to xcfx86s=(xcfx86s, xcex8s).
When receiving said signal the transducers 10-1 . . . 10-M of said transducer array 10 according to FIG. 10 generate analogue signals which are subsequently sampled by a multichannel analogue to digital converter 15. The sampled signals x1(n) . . . xM(n) are fed into an elementary beamformer 30. Said beamformer is represented by a filter bank comprising M FIR filters 35-1 . . . 35-M. An output signal yxe2x80x2(n) of said beamformer 30 is formed by adding the output signals xxe2x80x21(n) . . . xxe2x80x2M(n) of said FIR filters, respectively.
The beamformer 30 is adapted to steer the beam shape of the transducer array 10 to any particular look direction xcfx86i, by using a filter with an appropriate set of fixed filter coefficients. Said look direction xcfx86i of the beamformer does in general not necessarily coincide with the target signal source direction xcfx86s.
In the case that the signal is emitted from said signal source 20 into a noisy environment the sampled transducer signals x1(n) . . . xM(n) represent a noisy signal. At the same time, if the look direction xcfx86i to which the directivity of the beamformer 30 is correctly adjusted coincides with the target signal source direction xcfx86s, the noisy components in the transducer signals are substantially suppressed. The output signal yxe2x80x2(n) of the filter bank 30 estimates the signal emitted from said signal source 20; i.e. the signal to noise ratio SNR or any other appropriate measure of quality of the filtered signal yxe2x80x2(n) is maximized. Due to the fact that the beamformer 30 in FIG. 10 comprises only one set of fixed filter coefficients, the successful use of said system requires apriori knowledge about the target signal source direction xcfx86s. In order to enhance the beamformer output signal yxe2x80x2(n), it is necessary to align the look direction xcfx86i of the beamformer with said target signal source direction xcfx86s by providing an appropriate set of fixed filter coefficients.
The sequential alignment of the elementary beamformer""s directivity to several predetermined look directions xcfx86i, i=1 . . . N, e.g. to track a moving signal source, a respective set of fixed filter coefficients for each predetermined look direction xcfx86i has to be stored in a memory and implemented in said filter before usage. This would be very ineffective. Consequently, the system according to FIG. 10 is not appropriate for processing signals emitted from different signal sources located in different target directions xcfx86s,p p=1 . . . P with respect to the transducer array 10.
FIG. 12 shows an advanced beamformer 30xe2x80x2 capable to suboptimally overcome the disadvantages of the elementary beamformer. The advanced beamformer 30xe2x80x2 comprises Nxe2x89xa71 filter banks 30-1 . . . 30-N and is thus adapted to simultaneously calculate the beamformer output signals yxe2x80x21(n) . . . yxe2x80x2N(n).
Each of said filter banks comprises M finite impulse response FIR-filters for respectively receiving the same input signal vector x(n)=[x1(n) x2(n) . . . xM(n)], which is the output of the multi-channel analogue to digital converter 15. The M filters 35-i-1 . . . 35-i-M in each of the i=1-N filter banks are respectively characterized by the individual set of fixed filter coefficients with the result that each filter bank is adapted to one out of N look directions xcfx86i, i=1 . . . N.
The output signals of the beamformer in FIG. 12, i.e. the output signals of the filter banks 30-1 . . . 30-N, can be expressed as                                                         y              i              xe2x80x2                        ⁡                          (              n              )                                =                                    ∑                              j                =                1                            M                        ⁢                                          ∑                                  k                  =                  0                                                                      L                                          i                      ,                      j                                                        -                  1                                            ⁢                                                h                                      i                    ,                    j                    ,                    k                                                  ⁢                                                      x                    j                                    ⁡                                      (                                          n                      -                      k                                        )                                                                                      ,                  i          =                      1            ⁢            …            ⁢                          xe2x80x83                        ⁢            N                          ,                            (        1        )            
wherein Li,j is the length of the filter 35-i-j having fixed filter coefficients hi,j,k, k=0 . . . Li,j. Assuming Li,j=L, for all i=1 . . . N and j=1 . . . M the ith output signal yxe2x80x2i(n) can be rewritten in the form                                                         y              i              xe2x80x2                        ⁡                          (              n              )                                =                                    ∑                              j                =                1                            M                        ⁢                                          ∑                                  k                  =                  0                                                  L                  -                  1                                            ⁢                                                h                                      i                    ,                    j                    ,                    k                                                  ⁢                                                      x                    j                                    ⁡                                      (                                          n                      -                      k                                        )                                                                                      ,                  i          =                      1            ⁢            …            ⁢                          xe2x80x83                        ⁢                          N              .                                                          (        2        )            
One of the signals yxe2x80x2i(n), i=1 . . . N, can be selected as a desired output signal representing the best estimate of the signal being emitted towards the transducer array.
Said selection is done by a mixer 60 which preferably selects P signals out of said N beamformer output signals yxe2x80x2i(n) i=1 . . . N. Said P selected signals may represent P signals yp(n) for p=1 . . . P emitted from P different signal sources 20-1 . . . 20-P in respective target signal source directions xcfx86s,p for p=1 . . . P when the respective look directions xcfx86i of the beamformer output signals come at least close to said target signal source directions xcfx86s,p. Moreover, the mixer 60 can combine one or more beamformer output signals yxe2x80x2i(n) representing said signal sources 20-1 . . . 20-P together in one output signal yp(n) which may be desired e.g. in teleconferencing. In a simple implementation, when switching operation is preferred between different look directions, the mixer can be replaced with a multiplexer. The output signal selection is done by a beamformer output control unit 70 that provides a control signal to the mixer 60. In general the control unit 70 instructs the mixer 60 to select that beamformer output signal yxe2x80x2i(n) which has the best filtering performance, e.g. the look direction xcfx86i that comes closest to the direction xcfx86s,p in which the desired signal has actually been emitted.
The prior art system according to FIG. 12 comprising M transducers 10-1 . . . 10-M and M*L finite impulse response FIR filters 35-i-1 . . . 35-i-M of length L needs to keep up to
Mempri(S)=M*L*Nxe2x80x83xe2x80x83(3)
coefficients in a memory in order to filter Sxe2x89xa6N signals in parallel. In that case the computational load is defined by at least
Loadpri(S)=M*L*Sxe2x80x83xe2x80x83(4)
multiply-and-accumulate operations per sampling interval to form the S simultaneous beamformer output signals yxe2x80x2i(n). Moreover, there are only a maximum number of N predetermined fixed look directions available in the advanced beamformer 30xe2x80x2, being formed by the N filter banks.
To the contrary, a signal may also be emitted in a target signal source direction xcfx86s,p which does not coincide with any of said predetermined N look directions xcfx86i, i=1 . . . N. In this case the operation of the system would only be suboptimal, wherein the degree of suboptimalism depends on the difference between the actual target signal source direction xcfx86s,p in which the signal is emitted from the signal source and the look direction xcfx86i to which the beamformer is adjusted.
The prior art system according to FIG. 12 has the disadvantage that for each desired look direction xcfx86i of the beamformer a separate filter bank operating with an individual set of fixed filter coefficients has to be provided. Further, the system according to FIG. 12 enables only suboptimal receipt and restoration of signals being emitted in a target signal source direction xcfx86s,p which does not exactly coincide with any of the predetermined look directions xcfx86i of the beamformer, defined by the fixed coefficients.
This invention utilizes the theory of so called 1-dimensional (1-D) polynomial filters. It is known in the art that coefficients of a variable 1-D FIR filter can be represented as a polynomial. Polynomial based filters have been applied for variable delays (Farrow C. W., xe2x80x9cA continuously variable digital delay elementxe2x80x9d, ISCAS -88, pp. 2641-2645, 1988.), fractional delay filtering (Laakso T. I., Vxc3xa4limxc3xa4ki V., Karjalainen M., Laine U. K., xe2x80x9cSplitting the unit delayxe2x80x94tools for fractional filter designxe2x80x9d, IEEE Signal Processing Magazine, pp. 30-60, January 1996.), digital receivers (Tuukkanen V., Vesma J., Renfors M., xe2x80x9cCombined interpolation and maximum likelihood symbol timing in digital receiversxe2x80x9d, 1997 IEEE 6th International Conference on Universal Personal Communications Record, Vol. 2, pp. 698-702, 1997), digital modems (Erup L., Gardner F. M., Harris R. A., xe2x80x9cInterpolation in digital modemsxe2x80x94Part II: implementation and performancexe2x80x9d, IEEE Trans. on Comm., Vol. 41, No. 6, pp. 998-1008, June 1993.), and modeling of acoustic tubes (Vxc3xa4limxc3xa4ki V., xe2x80x9cDiscrete-time modeling of acoustic tubes using fractional delay filtersxe2x80x9d, Ph. D. Thesis, Helsinki University of Technology, Otaniemi, 1995/Report 37, pp. 95-104, 1995). All these methods aim to adjust the delay characteristics of a single 1-D FIR filter without affecting the magnitude spectrum of the signal. Another design method for variable 1-D FIR filters with variable cut-off frequencies has been presented by Deng (Deng T.-B., xe2x80x9cWeighted least-squares method for designing arbitrarily variable 1-D FIR digital filtersxe2x80x9d, Signal Processing, Vol. 80, pp. 597-613, 2000).
The optimal filter structure presented by Farrow for continuously variable digital delay has been generalized for 1-D polynomial based FIR filters. It is known in the art that the so called Farrow structure can be used to implement 1-D polynomial based FIR filters that support adjusting of filter characteristics using a single control parameter.
In order to steer the look direction of the elementary beamformer in FIG. 10, it would be possible to preprocess the transducer signals utilizing variable delay filters. However, this type of implementation would suffer from several drawbacks: optimal filtering characteristics of the elementary beamformer would be degraded when the look direction differs from the designed look direction of the original elementary beamformer, implementation of good enough 1-D fractional delay FIR filter would require considerably high filter orders, and each transducer signal would require a separate control parameter that would be dependent on each other.
In view of that situation it is the object of the invention to improve a system and method for processing a signal being emitted into a noisy environment in the way that clear signal reception is achieved for any target signal source direction xcfx86s with only a minimum of computational effort and memory capacity.
The object is solved for the system as described above in the way that a beamformer beam shape control system is provided for generating a control signal t(n) representing at least one physically relevant parameter for said target signal source, in particular said target signal source direction xcfx86s, that said filter coefficients of the beamformer are adjustable and that a filter coefficient generator means is provided for generating said filter coefficients in response to said control signal t(n) such that the beamformer has a predetermined filtering characteristic for said target signal source at said target signal source direction xcfx86s.
The beamformer beam shape control system includes means to detect all relevant parameters for said target signal source in relation to the transducer array, in particular said target signal source direction xcfx86s. According to the invention the beamforming filtering characteristic is exactly aligned to said parameters, in particular with said target signal source direction.
It is the main advantage of the system that the filter coefficients of the beamformer are not fixed but adjustable. The adjustable filter coefficients enable efficient calculation of multiple beamforming filters and thus the system is easily adjustable to the above mentioned physical parameters of a specific signal source. In particular the adjustable filter coefficients enable the system to continuously and smoothly steer the look direction of the beamformer. Expressed in other words, the system is adapted to point the best selectivity towards any desired target source direction and not only towards one of the predetermined target directions, as provided by the advanced beamformer known in the prior art. I.e. infinite accuracy is provided for pointing the look direction of the beamformer towards an arbitrary location of any desired signal source requiring only small memory and computational power. In that way an optimal predetermined filtering characteristic for any target signal source direction can be established.
The signal source emitting the signals received by the system is not restricted by its nature or by its location in space nor by the frequency of the signals generated by them. Consequently, there is the advantage that the system can be adapted to any kind of signals issued from the signal source.
Due to the automatic steering of the directivity of the beamformer or the transducer array in response to said control signal t(n) the system is very suitable for hands-free communications.
According to all embodiments of the invention it is advantageous that the beamformer having said adjustable filter coefficients has a polynomial filter characteristic. Such a characteristic enables simple calculation of the beamformer output signals.
It is advantageous that the filter coefficients are decomposed into fixed and variable filter parameters. Only the fixed filter parameters for one adjustable beamforming filter have to be stored in a memory and thus less memory capacity is required in comparison with the prior art system. Moreover, only the variable filter parameters have to be changed in order to steer the directivity of the beamformer and thus computational effort is reduced.
Preferably, the number of variable filter parameters is less than the number of fixed filter parameters. In that case the advantages of reduced memory capacity and computational effort are supported.
Advantageously, the adjustable filter coefficients of said polynomial filter characteristic are approximated by the following equation:
hj,k(Di)=a0(j,k)F0(Di)+a1(j,k)F1(Di)+ . . . aT(j,k)Fr(Di),
wherein at(j,k) are the fixed filter parameters,
wherein Di is a vector of variable filter parameters,
represented by said control signal t(n); and
wherein Ft(Di) are functions of said vector Di. This interpolation polynomial hj,k(Di) enables an easy change of the filter coefficients with insignificant computational complexity.
Said vector of variable filter parameters Di may be a single variable modeling an optimal polynomial filter for a trace of target signal source directions xcfx86s in space. More flexible source tracking in space would be possible by using a vector Di of three variables, e.g. representing the distance r, azimuth xcfx86, and elevation xcex8 of the target signal source direction xcfx86s.
In addition to the geometrical parameters, said vector of variable filter parameters Di may also represent one of the physically relevant parameters like background noise spectrum, desired signal bandwidth, signal spectrum, beam shape, physical properties of the medium such as temperature, system parameters such as camera control, or a combination thereof.
The functions are preferably Taylor functions because Taylor functions are easy to apply. However, the functions may also be Chebyshev functions in order to support optimization to minimize the maximum error.
According to a first embodiment of the invention the system comprises a memory for storing the fixed filter parameters at(j,k), the filter coefficient generating means for adjusting said filter coefficients according to equation hj,k(Di)=a0(j,k)F0(Di)+a1(j,k)F1(Di)+ . . . +ar(j,k)Fr(Di), and a signal processing means for processing the transducer output signals using said filter coefficients hj,k(Di).
Advantageously, said signal processing means is adapted to generate a beamformer output signal yxe2x80x2i(n), i=1 . . . N, by using equation             y      i      xe2x80x2        ⁡          (      n      )        =            ∑              j        =        1            M        ⁢                  ∑                  k          =          0                          L          -          1                    ⁢                                    h                          j              ,              k                                ⁡                      (                          D              i                        )                          ⁢                                            x              j                        ⁡                          (                              n                -                k                            )                                .                    
Processing the signals yxe2x80x2i(n) according to said equation has the advantage that the adaptation of the system to any arbitrary target direction can be supported by only adjusting said vector of variable filter parameters Di in response to said control signal t(n). Consequently, the computational effort for carrying out the adaptation is reduced to a minimum. Calculating several beamformer output signals in parallel facilitates the detection of the most appropriate signal according to the predetermined decision criteria.
According to a second embodiment of the invention the beamformer comprises: at least two filter banks each of which receives and filters said M transducer output signals in order to generate an intermediate signal yxe2x80x3t(n), t=0 . . . T; at least one post filter for receiving said at least two intermediate signals yxe2x80x3t(n) in order to generate said beamformer output signal yxe2x80x2i(n) i=1 . . . N in response to said control signal t(n) wherein the filter coefficient generator means is implemented in the combination of said at least two filter banks and said at least one post filter.
Preferably, each of said at least two filter banks calculates the intermediate signal yxe2x80x3t(n) from the transducer output signals xj(nxe2x88x92k) according to the following equation:             y      t      xe2x80x3        ⁡          (      n      )        =            ∑              j        =        1            M        ⁢                  ∑                  k          =          0                          L          -          1                    ⁢                                    a            t                    ⁡                      (                          j              ,              k                        )                          ⁢                              x            j                    ⁡                      (                          n              -              k                        )                              
wherein at(j,k) are predetermined fixed filter parameters. Said post filter calculates the beamformer output signal yxe2x80x2i(n), i=1 . . . N from said intermediate signals yxe2x80x3t(n) according to the following equation:                     y        i        xe2x80x2            ⁡              (        n        )              =                  ∑                  t          =          0                T            ⁢                                    y            t            xe2x80x3                    ⁡                      (            n            )                          ⁢                              D            i                    t                      ;
wherein Di is a vector of variable filter parameters represented by said control signal t(n).
Advantageously, the filter banks contain said fixed filter parameters whereas the post filter contains said variable filter parameters. In that way a proper separation between the processing of the fixed and the variable filter parameters for aligning the beam shape of the beamformer is achieved.
In any embodiment of the invention the transducers are preferably arranged linearly, 2-dimensionaly or 3-dimensionaly in said transducer array 10. Furthermore, they may have an omni-directional, a bi-directional, or some more complicated directional sensitivity characteristics, or a combination thereof. In that way an optimal receipt of the signals is achieved for any kind of signal characteristics. Transducer sensitivity characteristics can be taken into account in the design procedure of the beamforming filters and thus they do not limit the invention.
The transducers in the transducer array are preferably embodied as microphones for receiving acoustical signals, as antenna elements for receiving electromagnetic signals, or as hydrophones for receiving underwater acoustical signals.
It is advantageous that the system comprises at least one mixer for receiving and further processing at least two of said beamformer output signals yxe2x80x2i(n). In that way particular target signal source directions or combinations thereof can be selected for further processing.
For canceling echo effects in the output signal yxe2x80x2i(n) of the beamformer the system advantageously comprises an echo cancellation unit.
The system advantageously comprises an echo cancellation control unit for controlling said echo cancellation unit in response to said control signal t(n). In that way the performance of said echo cancellation unit is increased.
For further canceling noise in the output signal yxe2x80x2i(n) of the beamformer the system advantageously comprises a noise cancellation unit.
The system advantageously comprises a noise cancellation control unit for controlling said noise cancellation unit in response to said control signal t(n). In that way the performance of said noise cancellation unit is increased.
The object of the invention is further solved by a method for processing a signal received from a target signal source into a noisy environment. The advantages of said method correspond to the advantages of the system as discussed above.